This invention is directed to a method and apparatus to effectively and efficiently split, image, and focus a laser beam, preferably an excimer laser for the purpose of laser assisted plating, onto a workpiece.
A preferred embodiment of this invention lies in the practice of laser assisted plating of electrical terminals. Typically, such terminals are stamped and formed from an electrically conductive metal strip and are attached to a carrier strip which is useful for strip feeding the terminals through successive manufacturing operations. The final manufacturing operation involves plating the electrical contact surfaces of the strip fed terminals with precious metal or semi-precious metal, such as gold or alloys thereof. Such metals are characterized by good electrical conductivity and little or no formation of oxides that reduce said conductivity. Therefore these metals, when applied as plating, will improve conductivity of the terminals. However, the high cost of these metals has necessitated precision deposition on the contact surfaces of the terminals, and not on surfaces of the terminals on which plating is not necessary. Precision deposition is therefore very critical to a cost effective operation. Further, the high cost of equipment, and the operation and maintenance thereof, requires efficiency in the plating procedures.
There are conventional selective plating practices which do not utilize lasers, but rather relate to apparatus which includes a rotating mandrel for guiding terminals through a plating solution. See U.S. Pat. Nos. 4,555,321 and 4,473,445. One difficulty associated with such apparatus is the ability to control the precious metal deposition to extremely small contact areas, and the close proximity of adjacent terminals on a carrier strip.
The introduction of laser technology to the plating process has resulted in new approaches to such process. U.S. Pat. No. 4,348,263 to Draper et al and directed to a process for surface melting of a substrate prior to plating, teaches a method of making an electrical contact by the steps of applying a first protective layer to a substrate, subjecting said protective layer and a portion of said substrate to melting by means of an electron beam or laser prior to the deposition. A related work by Draper, published in the Gold Bulletin, 1986, 19, entitled "Laser Surface Alloying of Gold," contains an illustrated showing on the mechanism of laser surface alloying by the use of focussed laser pulsing.
As applied herein, by way of example only, laser assisted plating may consist of improving the substrate prior to plating; precise removal of a plating resist to expose defined contact plating areas; or, laser plating in vacuum. The latter two techniques are taught, respectively, in co-pending application, Ser. No. 180,417, and assigned to the assignee herein, and U.S. Pat. No. 4,427,723 to Swain. Said co-pending application teaches a method for the selective plating of a metal substrate, which method includes the steps of selecting a laser wavelength which is strongly absorbed by the metal substrate, i.e. reflectivity of less than about 70%, choosing a polymer based plating resist having a low optical coefficient of absorption to a laser wavelength, typically between about 248 to 360 nm, curing said resist, preferably subjecting selective areas of said resist to a single excimer laser shot, having a wavelength between about 248 to 360 nm, to heat the metal substrate and thereby cause ablative removal of the resist over the selective areas of said substrate, and subjecting said exposed portions of said substrate to metal plating.
The Swain patent teaches a method and apparatus for vacuum depositing and annealing, wherein a coating material is evaporated by the action of a laser beam, while the substrate to be coated is scanned by another laser beam to cause localized heating and depositing of the coating material on the substrate.
In such practices utilizing an excimer laser or other such laser producing a characteristically broad output beam, whose size and shape at the laser exit are determined mainly by the discharge electrodes and hence are beyond control of the user, there has been no apparent attempt to direct or transmit the laser beam in a way to substantially utilize the full energy of the laser beam. Specifically, in material processing with excimer lasers, a specific interaction area is generally defined by utilizing an aperture to restrict the beam. When an aperture with a small opening is used, a large portion of the beam is wasted, since excimer lasers typically have large-area output beams. The alternative of focussing the entire beam onto the area of a small aperture can damage the aperture and optics and will increase the beam divergence. A mask with multiple apertures placed within the beam cross section can give a degree of parallel processing, but the fraction of light wasted between apertures may still be large.
In copending application U.S. Application Ser. No. 273,381, , a system is described to better utilize the energy of the laser. Such system comprises an excimer laser emitting a beam having a predetermined cross-section, one or more elongated light-guides, i.e. rods, strips or fibers, to direct said beam, where the cross-sections thereof are sufficient to completely fill said laser beam cross-section, and means for shaping and focussing said beam to a reduced image on a workpiece.
The present invention represents a further improvement thereover by the use of an efficient holographic beam splitter and imaging technique for laser assisted plating. The use of holograms or holographic components are of recent origin, but are nevertheless known. For example, U.S. Pat. No. 4,170,396 to Kraft discloses a holographic component including a carrier member and a plurality of sub-holograms formed thereon. The component may create a division of a beam of spaciously coherent radiation into a plurality of different partial beams. The component may be used in a holographic storage system, or in an optical material processing system which uses a laser to solder or weld. A YAG laser of continuous operation is used as a source. The component divides a beam for soldering or welding at discrete separate point.
By way of further example, U.S. Pat. No. 4,359,259 to Horner et al discloses a holographic fiber optic directional coupler for separating an incoming beam into a plurality of outgoing beams and for combining a plurality of incoming beams into a single outgoing beam in the reverse direction. The system is used for an optical fiber communication system.
Neither of the latter patents teach an efficient system which utilizes a holographic laser beam splitter and imaging technique in a laser assisted plating operation. More particularly, such patents fail to teach or suggest the use of a pulsed laser which is synchronized to the speed of a moving workpiece, such as electrical terminals joined by a carrier strip, whereby multiple terminals may be treated concurrently.